1. Field of the Invention
This invention relates to an article surface defect detection system and, more particularly, to a method of and video system for discriminating between different types of light-reflective surface areas on articles, such as involved in ascertaining the presence and absence of, as well as condition of, circuit paths and satisfactory versus unsatisfactory soldered connections on printed circuit boards.
2. Description of the Prior Art
The simplest way to ascertain whether all of the areas that should be soldered have been on a printed wiring board and, if soldered, whether the soldered connections are satisfactory or not, is to visually examine each connection, preferably through a magnifying lens, with special attention being given to the surface area profile of each connection. More specifically, the profile of a typical satisfactory soldered connection should appear as a relatively smooth and uniformly contoured solder fillet. An examination of the profile of each soldered connection (fillet) can also provide quite accurate information as to whether or not any clinched lead end of a solder-connected component is present and, if so, whether it is properly oriented, cut to the desired length, and if of the clinched type, biased firmly against the wiring board.
In addition to examining a printed wiring board for any possible solder defects, it is also normally important that the printed circuitry per se be examined (at least visually scanned) for any possible defects therein, such as open circuits, or solder bridges, or icicles, that can produce short circuits between adjacent circuit paths.
It becomes readily apparent that when a direct visual examination procedure is employed, it is not only very time consuming and tedious, but involves considerable eye strain, notwithstanding the use of any magnifying lens. Compounding the problem of eye strain is the fact that with respect to circuit boards having very high density circuitry fabricated thereon, the spacing between adjacent soldered connections and circuit paths is extremely close, often being less than several mils. This is particularly true in many microminiaturized electronic circuit packs employed in diverse types of electronic equipment and systems. High density printed circuitry, of course, also has a tendency to produce substantial light-reflected glare off of the relatively smooth surfaces thereof. Such glare can very readily establish blind spots that can significantly contribute to the defect-identification error rate, as well as contribute to operator fatigue.
Such problems, at least with respect to the examining of printed circuit boards, has been obviated to a great extent heretofore through the utilization of commercially available electronic scanners that employ either an incandescent light source, or a laser beam, in conjunction with a reflected light-receiving photodiode matrix, for example, to differentiate between different light-reflective surface characteristics on the board. In such scanner systems, the reflected light pattern read out of the photodiode matrix, representative of the scanned surface of the test specimen, is normally compared against a standard reference pattern previously read out of the matrix. Such a system is quite effective in discriminating between flat, conductive printed circuit paths and the non-conductive surfaces of a circuit board. Systems of the described type, however, are not particularly useful after component assembly and soldering, as the resulting solder fillets, in terms of size and profile, present an infinite number of light-reflective variables and, hence, an infinite number of required matrix patterns, in order to reliably examine an entire circuit board, not to mention making a determination of even one satisfactory versus unsatisfactory soldered connection. The particular orientation of clinched lead ends also presents an infinite number of variables to any reflected light-responsive scanning system.
British Pat. No. 727,480 discloses a TV or video system for viewing an entire object, as distinguished from a selected surface thereof, in the presence of scattered light caused, for example, by minute creatures or particles under water, or smoke particles in the atmosphere. The apparatus includes a light source for illuminating the object with a beam or beams of substantially parallel light projected downwardly onto the object (as distinguished from across a given surface to be viewed) in a direction substantially perpendicular to the axis of a TV camera. A light polarizing filter is suitably oriented in the path of the light (either between the light source and object or between the latter and the camera) to reduce the effect of scattered light on the camera. As arranged and operated, this prior video system can only discriminate between the environment (with the minute particles, for example, causing the scattered light) and the entire object which is intended to be seen as clearly as possible.